As the method for heat-treating a high-temperature steel material subsequent to the hot-rolling step by rapid cooling or gradual cooling, there are known, for example, in the case of a wire rod, a method in which air, mist, water or the like is sprayed onto a hot steel material, a method in which a hot steel material is immersed in a metal bath of salt or lead, a method in which a hot steel material is immersed in a liquid such as warm water, cold water or oil, and a method in which a hot steel material is immersed in a fluidized and stirred warm liquid, as shown in Japanese Unexamined Patent Publication No. 57-9826. These methods are utilied according to the intended use, respectively, and although most of these methods are unifunctional, they have excellent characteristics. However, to cope with one of the requirements in the iron and steel production process, that is, multiple variety-small lot production, a plurality of cooling apparatus must be disposed, and thus the production process is complicated and the installation cost increased.
As a means for overcoming this disadvantage, there has been developed a multi-functional heat treatment system as introduced in the Journal of Japanese Association of Metallurgy, 25, 6 (1986), page 559. This system is characterized in that the air-blast method is adopted at a cooling speed lower than 10.degree. C./sec and the immersion method is adopted at a cooling speed higher than 10.degree. C./sec. The installation has a two-line structure of upper and lower lines, and a cooling medium-circulating apparatus is disposed in the lower line as the immersion line for cooling a hot steel material in the fluidized and stirred state. A temperature-maintaining cover and a blower are arranged in the air-blast line as the upper line. Each line obtains a desired cooling speed of 2.degree. to 100.degree. C./sec.
In a controlled cooling of a spring steel or high-carbon steel wire rod, especially a spring steel, if immersion cooling is carried out in hot water having a temperature close to the boiling point, the cooling speed is too high and a supercooled texture is formed, and if natural, gradual cooling in the open air is adopted, the cooling speed is not uniform and ferrite decarburization is caused in a portion where the cooling speed is low. Accordingly, an air-blast cooling method comprising blowing air onto a hot steel material is generally adopted as the means for obtaining an appropriate cooling speed at the immersion cooling in hot water and an intermediate cooling speed at the natural, gradual cooling in the open air. However, when a non-concentric wire ring coiled at a laying head is continuously placed and cooled on a conveyor, the lapping degree (density) of each ring is changed from the center of the ring toward both the side portions, and therefore, a measure is adopted for changing the air blast quantity according to the lapping degree of the ring. However, in the conventional method, wherein air is supplied from below, cooling is insufficient in the lapped portion of the ring and a uniform cooling cannot be attained, and the dispersion of the cooling speed in one ring is larger than the dispersion of the cooling speed in a material treated by immersion in hot water. Accordingly, the deviation of mechanical properties such as tensile strength is increased, and an adverse influence is imposed on the wear of a tool and dimension precision at the secondary processing, and a problem of an occurrence of bending at the straightening step arises.
As the cooling method for homogeneous patenting, there is known a method in which a hot steel material is immersed and cooled in a strongly stirred fluid medium comprising a gas and warm water. In this method, however, since the cooling medium is in the strongly stirred state, a fine wire having a diameter of about 5.5 mm is oscillated while supplied by a conveyor, and problems arise at the transfer step and a partial peeling of scale occurs, with the result that a roughening of the surface is caused at the mechanical descaling or pickling step. Furthermore, since a large quantity of air is blown, the running cost is increased because of the presence of a blower and a cooling medium-circulating apparatus is necessary, and therefore, the installation scale is increased and the system becomes complicated, and the equipment cost is drastically increased.